How to apply mutiple images to the material of UV Unwraped mesh
What I'm going to show you one possible way to to join different textured objects into a single object with a single texture.

Here's the roadmap:



Assuming that we are starting with two separate object with assigned textures as in the picture (Texture A for the cube, Texure B for the sphere):



First step is to create a material based on the assigned texture to each object.

Part One - Creating basic textured materials in Blender Internal


Select one object (Cube in this case)
Go to the Material tab
Create new material (rename if wanted)
Go to the Texture tab
Create a new Material texture
"Type" must be "image or movie"
Choose the right texture from the built in browser (it will show you all the textures related to your file, so images you based your UV to must shown in the list)




A basic material where the texture is by default applied to the diffuse color of the shader has been created. Repeat the same processing for the other object.

The process is better explained in this video, or in this related answer

Part Two - Join texture in a texture atlas

First step is about obtain a single object


Merge all objects by selecting all of them and pressing ctrl+j


Now there is only one object with several materials applied. Blender conserves the information about what material was assigned to each face during the join operation.

By going in the "Object data" tab, you can see that there is only a UV map. This is because both of my objects had an UV map called "UVMap", so in the merging process the have been joined under the same name. Tey are now overlayed...but they still act as before in their proper texture space, as you can see in the viewport. If we want a single texture, we must avoid overlaying, so:


Create a new UVMap by clicking the "+" button.


This new UV, called UVMap.001, will contain the uv info about the texture atlas we are going to create. Select this map, but don't set as "Active for rendering".

You can see here what UVMap you are working on:



In the UV/Image editor, clic on the "X" to clear the editor and than create a new image. This will be the image to bake to.
You can rearrange the UVs as you wish or let blender do the job by clicking "Pack island":



Go to the "Render" tab, in the Bake panel, bake mode, select "textures" in order to bake the textures of your object's materials (Texture A and Texture B) to the Image (Untitled.001) which is below your selected UV map (UVMap.001).